Ceramic materials with nano-sized inclusions have received increased interest during the recent years. The interest lies both in the possibilities to make fundamental studies on new properties associated with the nano-size and in applications in e.g. optic absorption and non-linear materials, catalysts and magnetic materials. One might also envisage applications of these metal-ceramic composites as precursors for nano-structured construction ceramics with excellent toughness/hardness properties. Most promising routes for preparation of nano composites of large scales and complex systems goes through solutions of inorganic salts or metal-organic compounds due to their cost effectiveness and possibilities in yielding complex compositions.
In the M-Al2O3 system, the materials are usually prepared from precursors first yielding NiO and/or NiAl2O4, followed by reduction with hydrogen gas to Ni—Al2O3. This is an inefficient approach since the reduction of especially the binary oxide is difficult and requires high temperatures or long times under reducing atmospheres. The metal formed during the reduction is then much more mobile on the alumina surface and larger metal particles are easily formed as well as very wide metal particle size distributions.
Supported nickel catalysts are among the most active for the CO2 reforming of CH4 to produce syngas, i.e. CO and H2. However, there is a well-documented problem of carbon deposition (poisoning), which deactivates the catalyst, when the nickel-particles are too large.
Metal nano-inclusion ceramic materials (NIM:s) with the metallic particles in the 1-20 nm size range have great potential to become the leading materials in a very wide range of materials, in e.g., mechanically or thermally durable coatings or compacts for cutting tools and light engines (high-performance structural ceramics), selective optical absorption devices for solar heat-absorption, catalysts, highly non-linear optics for photonic switches and frequency converters, transformers, magnetic sensors, temperature stable colour coatings and as substrates for carbon nano-tube growth for use as cold cathode emitters and nano-electronics.
Despite this great potential in a wide area of high- and medium-tech applications, there is still very limited knowledge concerning how to successfully tailor the sizes, contents and shapes of the metal particles and the morphology of the matrix, e.g. as thin films, compacts or porous materials. The methods used for preparation include a variety of physical and chemical techniques. However, the physical and chemical vapour based routes suffer from the drawback that they cannot be used for large-scale applications of NIM-film and NIM-powder preparation and they are comparatively expensive and also require sophisticated vacuum equipment. These techniques are normally restricted to small surfaces, a few centimetres in size, due to limitations in vacuum chamber size and control of the deposition. The deposition is also relatively slow and is often dependent on the substrate.
The chemical routes to Al2O3 based NIM:s, have to a large extent been directed to powders only, and many of the processes reported use high temperatures and/or long annealing times and cannot be used in film preparation. Several processes have been published with the common feature that a salt based solution is gelled or precipitated and annealed in air at different temperatures, normally between 500° C. and 1200° C., yielding a ceramic material containing NiO and Al2O3 and/or NiAl2O4. This powder is then reduced under hydrogen-containing atmosphere during heat-treatment for several hours at ≦1000° C. [E. Breval et al., J. Mater. Sci., 27 1464-1468 (1992)]
Another common partly solution based route for powder preparation is to start with a slurry of Al2O3 powder and nickel containing solution. After evaporation of the solvent the obtained powder is then dried at varying temperatures, typically between 50 and 500° C., and then reduced by heating in H2 at temperatures≧500° C. [see W. H. Than, H. H. Wu, and T. J. Yang, J. Mater. Sci., 30 855-859 (1995), T. Sekino, T. Nakajima, S. Ueda, and K. Niihara, J. Am. Ceram. Soc., 80 1139-1148 (1997)].
Some of the above discussed processes are capable of producing small metal particles, but the control of the powder surface area and morphology is quite limited. There are complex processing routes available involving several steps including the use of the potentially dangerous hydrogen gas at high temperatures and long annealing times. The high temperature and long annealing times leads to a large energy consumption and makes the process expensive, and also reduces the number of possible applications.
Films of Al2O3-based NIM:s are usually made by sputtering techniques [see e.g.: M. Gadenne, P. Gadenne, M. T. Ramdou, J. P. Seagaud, H. Lassri, R. Krishnan, and C. Sella, Materials Science and Engineering, A168 257-261 (1993)]. There is one solution-based route reported. It uses spin-coating of a solution of nickel 2-ethyl-hexanoate and aluminium tri-sec-butoxide (which is a complex synthesis route by refluxing, centrifugation, vacuum evaporation, preformed with two high temperature steps), and the gel film formed is heated to 1200° C. in air to form the spinel NiAl2O4, which is then reduced by heating in H2 at 950° C. for 5 minutes. [G. T. Kraus, Y.-C. Lu, J. E. Trancik, D. M. Mitro, E. P. Giannelis, M. O. Thompson, and S. L. Sass, J. Appl. Phys., 82 1189-1195 (1997)].
JP 07-114048 discloses hyperfine particles of a metal selected from Fe, Co and Ni. The particles are incorporated into a matrix of a non-linear optical material. The matrix has an Al oxide phase and an amorphous alloy phase, consisting of the metal and Al. Metal particles≦50 Å can be incorporated, where the alloy phase consists of ≦50 wt % Al and Fe, Co or Ni. The optical material can be produced as a thin film by sputtering or other methods.
In U.S. Pat. No. 5,462,903 (CNRS) there is disclosed nano-composite powders of alumina and metal constituted of grains of micronic size. Each grain comprises a compact matrix of alumina, in which there are dispersed crystallites of metals or alloys, the size of which is less than 50 nm. The ratio by weight of metal/alumina is less than 30%.
So far, all routes to Ni—Al2O3 are producing only low nickel loadings, typically up to about 30%, but many applications, such as in magnetic applications and solar heat absorption applications, require very high metal loadings, typically 60-90%.
The compositions of nickel metal inclusions in alumina, Ni—Al2O3, and some of its compositionally modified variants are among the most versatile NIM:s. This type of NIM with different metal particle sizes and concentration (volume content of metal particles in the matrix alumina) in the forms of high surface area materials, films and compacts have many important areas of application.